1. Field of the Invention
The present invention generally relates to a fuel cell system and its method of operation.
2. Description of the Related Art
Typically, for a solid-state polymer type fuel cell, the optimum temperature for power generation is in the range of 70-90° C. When the fuel cell is started at sub-freezing temperatures, the water in the fuel cell stack is frozen, so that the reaction gases generally cannot be well fed to the catalyst layer, and the catalyst component of the electrodes cannot promote the electrochemical reaction, thus leading to deteriorated performance. In order to avoid this problem, the following method has been proposed as described in Japanese Kokai Patent Application No. 2006-100093. In that method, the residual water volume in the fuel cell stack is computed, and on the basis of the residual water volume and the internal temperature of the fuel cell stack, the maximum current that can flow through the fuel cell stack is computed, and the start-up current is restricted to the maximum current that can flow.
Also in that method, the residual water volume is determined from the volume of water purged from the interior of the fuel cell stack during the last shutdown cycle, the change in weight of the fuel cell stack, the fuel cell stack resistance, the fuel cell stack final load, and so on. However, when the residual water volume estimation device is carried onboard the vehicle, the device can occupy an undesired amount of space on the vehicle, which can be problematic.